This invention relates to a molding method for forming part-spherical acetabular cup shell or knee femoral components from powdered metal, preferably powdered Titanium (Ti).
Parts having simple geometries, such as flat plates, can easily be compression molded. Presently molding complex geometries from metal or polymer powder is typically done using either injection molding or isostatic pressing and usually not compression molding.
Injection molding can make complicated shapes with a well-designed mold, but the molds and molding equipment can be expensive. In addition, powder additives are used to allow the powder to more easily flow in the mold. These additives can interfere with obtaining a fully dense part and be difficult to remove from the part after molding. The powder feedstock also needs to be carefully selected for its flow characteristics and can be relatively expensive. Furthermore, injection molding is performed at lower pressures which cannot deform the metal powder and therefore requires a binding agent to compact a part that can be subsequently handled/post processed. Having a binder as well as a flow enhancing agent takes up space, (often up to about 20% of the compacted part mixture) and results in a large amount of dimensional change during the sintering consolidation.
Isostatic pressing typically involves a pressure-filled bladder pressing the powder against a rigid surface. Thus, tooling and equipment costs can be less than with injection molding, but additives (such as binders) are generally still necessary. A drawback to isostatic pressing is that the quality of the surface adjacent to the bladder is not as good as can be obtained with injection molding.
Compression molding one-dimensional objects, such as a hockey puck, from powder is simple because the resulting object is the same thickness in the direction of travel of the moveable ram of the compression mold. Flat multi-level parts such as gears are more difficult to mold and normally to achieve a relatively uniform density throughout, the part multi stage tooling and/or additional pressing rams need to be used. Molding a two-dimensional object, such as a hemispherical shell, from metal powder is more difficult because even if the wall thickness of the resulting object is uniform, the thickness measured in the direction of mold or ram travel or direction of compaction can vary. Thus for an acetabular shell the radial thickness between an inner and outer part-spherical surface can be constant but varies in a direction perpendicular to the equator.
What is needed is an economical method to mold two-dimensional shapes (such as a hemisphere) from powder. In order to be economical the method should be able to utilize inexpensive powder feedstock, require no binding agent to be mixed into the powder, require no additive to improve flow characteristics of the powder, require tooling and equipment that is relatively inexpensive compared to injection molding, and allows for better surface quality on all sides of the resulting part than is possible with isostatic pressing. In some cases a die wall lubricant is used to improve the life of the mold/tool and to improve the ejection of the part out of the tool. However, it is important to keep residuals out of the part when compacting Titanium due to its high reactivity during sintering. Compression molding can achieve these objectives. In the past, the problem has been that molding parts of different thicknesses, such as an acetabular cup shell, requires different amounts of compression in the direction of compression motion in different areas of the mold. This is difficult to achieve using a mold having only one moveable compression part.